Hitherto, various ultrasonic motors making use of piezoelectric effects have been proposed. Patent Document 1 indicated below discusses an ultrasonic motor including a stator, formed by affixing a circular ring-shaped elastic member to a circular ring-shaped piezoelectric vibrator, and a rotor, which contacts the circular ring-shaped elastic member. In the circular ring-shaped piezoelectric vibrator, a plurality of electrodes are formed on both surfaces of a circular ring-shaped piezoelectric member formed of piezoelectric ceramics. The plurality of electrodes include a first phase electrode group and a second phase electrode group. By applying high-frequency voltages having different phases temporally to the first phase electrode group and the second phase electrode group, two standing waves are generated. By combining the two standing waves to each other, a traveling wave is generated at a surface of the ring-shaped elastic member and a surface of the ring-shaped piezoelectric vibrator.
In an ultrasonic motor using a circular ring-shaped piezoelectric vibrator such as that discussed in Patent Document 1, it is difficult to manufacture the piezoelectric vibrator. That is, in order to excite a predetermined vibration, the piezoelectric member must be precisely formed into a circular ring shape. Therefore, it is necessary to polish piezoelectric ceramics with high precision so that the center of an outer circumferential edge and the center of an inner circumferential edge of the circular ring-shaped piezoelectric member are the same. Further, processing must be separately performed in polarization and electrode formation, thereby tending to increase costs.
In the ultrasonic motor discussed in Patent Document 1, since one surface of the circular ring-shaped elastic member is entirely covered by the circular ring-shaped piezoelectric vibrator, it is necessary to contact an opposite side of the circular ring-shaped elastic member with a rotor. That is, the surface that contacts the rotor is limited to one surface of the ring-shaped elastic member. Therefore, there are large design limitations.
On the other hand, Patent Document 2 indicated below discusses an ultrasonic motor including a stator shown in FIG. 12. In the stator 101 shown in FIG. 12, a circular ring-shaped elastic member 102 formed of, for example, metal is used. A plurality of projections 103 are formed at the lower surface of the elastic member 102 so as to be disposed in a dispersed manner in a circumferential direction. The plurality of protrusions 103 are provided by forming a plurality of slits 104 extending in a radial direction.
A plurality of first piezoelectric elements 105a and a plurality of second piezoelectric elements 102b are affixed to the upper surface of the circular ring-shaped elastic member 102. More specifically, an A-phase driving section 105A, including a portion in which four first piezoelectric elements 105a are disposed, and a B-phase driving section 105B, including a portion in which four second piezoelectric elements 105b are disposed, are provided. At the A-phase driving section and the B-phase driving section, standing waves whose phases differ from each other are temporally excited. By combining these standing waves, a traveling wave is formed. When the wavelength of the traveling wave is represented by λ, the A-phase driving section 105A and the B-phase driving section 105B are disposed at a 3λ/4 interval at one side and at a λ/4 interval at the other side.
In manufacturing the ultrasonic motor discussed in Patent Document 2, the plurality of piezoelectric elements 105a and 105b having rectangular planar shapes are affixed to the upper surface of the circular ring-shaped elastic member 102 that is formed of, for example, metal and that is easily processed. Therefore, it is not necessary to perform complicated processings of piezoelectric ceramics.
[Patent Document 1] Japanese Examined Patent Application Publication No. 1-17354
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 11-187677
In each of the A-phase driving section and the B-phase driving section, a standing-wave 5-wave whose wave number is 5 is excited. In the specification, when the wave number of the standing wave that propagates through a ring-shaped portion in a circumferential direction is S (S is a natural number), the wave is called an S wave. In order to efficiently excite each of these S harmonic waves, it is desirable that a circumferential length of an area where excitation occurs at one piezoelectric element 105a or one piezoelectric element 105b, that is, the size along a propagation path of a traveling wave be λ/2, and that the entire area of the portion corresponding to the circumferential length λ/2 be excited.
However, in the ultrasonic motor discussed in Patent Document 2, gaps indicated by arrows C are formed between the rectangular piezoelectric elements 105a that are adjacent to each other and the rectangular piezoelectric elements 125a that are adjacent to each other. Therefore, excitation efficiency cannot be increased.
In the ultrasonic motor discussed in Patent Document 2, in order to reduce the gaps C, the plurality of piezoelectric elements 105a and 105b may have the shape of a fan or a trapezoidal shape. However, in this case, the piezoelectric elements cannot be efficiently formed at a low cost, thereby increasing costs.
In addition, in the ultrasonic motor, a holding structure of the stator 101 tends to be complicated. In the stator 101, the plurality of piezoelectric elements 105a and 105b are provided at the upper surface side of the circular ring-shaped piezoelectric member 102, and the rotor is made to contact the projections 103 at the lower surface side to perform a driving operation. Therefore, in order to hold the stator 101, it is necessary to connect one end of a connection portion to a side surface of the circular ring-shaped elastic member 102, and to connect the other end of the connection portion to a holding portion. Therefore, the holding structure including, for example, the connection portion tends to be complicated.
In addition, in the ultrasonic motor, the rotor must be brought into contact with a surface of the elastic member 102 at a side opposite to the side where the piezoelectric elements 105a and 105b are placed. Therefore, the surface that contacts the rotor is limited to one surface of the elastic member 102.